Generating random numbers in Java

Java provides three ways to generate random numbers using some built-in methods and classes as listed below:

• java.util.Random class
• Math.random method : Can Generate Random Numbers of double type.
• ThreadLocalRandom class

1) java.util.Random

• For using this class to generate random numbers, we have to first create an instance of this class and then invoke methods such as nextInt(), nextDouble(), nextLong() etc using that instance.
• We can generate random numbers of types integers, float, double, long, booleans using this class.
• We can pass arguments to the methods for placing an upper bound on the range of the numbers to be generated. For example, nextInt(6) will generate numbers in the range 0 to 5 both inclusive.

// A Java program to demonstrate random number generation
// using java.util.Random;
import java.util.Random;

public class generateRandom{

    public static void main(String args[])
    {
        // create instance of Random class
        Random rand = new Random();

        // Generate random integers in range 0 to 999
        int rand_int1 = rand.nextInt(1000);
        int rand_int2 = rand.nextInt(1000);

        // Print random integers
        System.out.println("Random Integers: "+rand_int1);
        System.out.println("Random Integers: "+rand_int2);

        // Generate Random doubles
        double rand_dub1 = rand.nextDouble();
        double rand_dub2 = rand.nextDouble();

        // Print random doubles
        System.out.println("Random Doubles: "+rand_dub1);
        System.out.println("Random Doubles: "+rand_dub2);
    }
}

Output:

Random Integers: 547
Random Integers: 126
Random Doubles: 0.8369779739988428
Random Doubles: 0.5497554388209912

2) Math.random()
The class Math contains various methods for performing various numeric operations such as, calculating exponentiation, logarithms etc. One of these methods is random(), this method returns a double value with a positive sign, greater than or equal to 0.0 and less than 1.0. The returned values are chosen pseudorandomly. This method can only generate random numbers of type Doubles. Below program explains how to use this method:

// Java program to demonstrate working of
// Math.random() to generate random numbers
import java.util.*;

public class generateRandom
{
    public static void main(String args[])
    {
        // Generating random doubles
        System.out.println("Random doubles: " + Math.random());
        System.out.println("Random doubles: " + Math.random());
    }
}

Output:

Random doubles: 0.13077348615666562
Random doubles: 0.09247016928442775

3) java.util.concurrent.ThreadLocalRandom class
This class is introduced in java 1.7 to generate random numbers of type integers, doubles, booleans etc. Below program explains how to use this class to generate random numbers:

// Java program to demonstrate working of ThreadLocalRandom
// to generate random numbers.
import java.util.concurrent.ThreadLocalRandom;

public class generateRandom
{
    public static void main(String args[])
    {
        // Generate random integers in range 0 to 999
        int rand_int1 = ThreadLocalRandom.current().nextInt();
        int rand_int2 = ThreadLocalRandom.current().nextInt();

        // Print random integers
        System.out.println("Random Integers: " + rand_int1);
        System.out.println("Random Integers: " + rand_int2);

        // Generate Random doubles
        double rand_dub1 = ThreadLocalRandom.current().nextDouble();
        double rand_dub2 = ThreadLocalRandom.current().nextDouble();

        // Print random doubles
        System.out.println("Random Doubles: " + rand_dub1);
        System.out.println("Random Doubles: " + rand_dub2);

        // Generate random booleans
        boolean rand_bool1 = ThreadLocalRandom.current().nextBoolean();
        boolean rand_bool2 = ThreadLocalRandom.current().nextBoolean();

        // Print random Booleans
        System.out.println("Random Booleans: " + rand_bool1);
        System.out.println("Random Booleans: " + rand_bool2);
    }
}

Output:

Random Integers: 536953314
Random Integers: 25905330
Random Doubles: 0.7504989954390163
Random Doubles: 0.7658597196204409
Random Booleans: false
Random Booleans: true

References:
https://docs.oracle.com

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